An informational notice recently issued by the Nuclear Regulatory Commission (NRC) cautions nuclear power plant operators to look out for moisture degradation of structures and components used to store spent nuclear fuel in dry casks.

The NRC cited a case in October 2010 when a cask seal pressure monitoring alarm was set off at Peach Bottom Atomic Power Station and rust was later discovered on the cover and access plate of the bolted-closure cask system, even though it had a double-mechanical O-ring seal that was intact and sealed. Signs of moisture were also found around most of the bolt-lid holes and bolts. A helium leak test later identified that the outer sealing surface of the main cask lid seal was leaking at a rate greater than allowed by technical specifications.

A root cause evaluation concluded that the seal leakage was caused by corrosion of the outer portion of the cask lid seal from water infiltration through the access plate in the protective cover. "The water infiltration caused galvanic corrosion of the outer portion of the cask lid seal due to the presence of moisture at the interface of the aluminum-clad cask lid seal and the stainless steel clad cask body sealing surface. The presence of the moisture at the interface of the two dissimilar metals set up a galvanic cell that caused the aluminum to corrode and allowed helium to leak through the outer portion of the cask lid seal," the NRC said.

The NRC also described a case involving horizontal storage modules (HSM) from Three Mile Island’s Unit 2. Operators at the nuclear plant noted cracks in the HSMs in 2000, but in 2007, the licensee saw more cracking, crazing, and spalling, as well as "increased efflorescence" on the HSM surfaces. Later, operators found that 28 of their 30 HSMs had cracks, "mostly emanating from the anchor bolt blockout holes with widths up to 0.95 centimeters (0.38 inches)," the NRC said.

An evaluation concluded that water had entered the anchor bolt blockout holes on the roof of the HSM, and subsequent freeze and thaw cycles had initiated the crack formation.

"These examples show the importance of periodically monitoring the physical condition of a spent nuclear fuel storage system. By obtaining baseline measurements and performing periodic evaluations, accelerated degradation can be detected before the structures and components of a storage system are unable to perform their intended function, and corrective actions can be implemented," the NRC said.

Sources: POWERnews, NRC

Note: This story was originally published on May 1

Sonal Patel, Senior Writer (@POWERmagazine, @sonalcpatel)